recoverd: When calculating rebalance candidates don't consider flags

recoverd: When calculating rebalance candidates don't consider flags

This is really a check to see if a node is already hosting IPs.  If
so, we assume it was previously healthy so it isn't considered as a
rebalance candidate.  There's no need to limit this to healthy node,
since this is checked elsewhere.

Due to this the variable newly_healthy is renamed everywhere to
rebalance_candidates.

The mask argument is now completely unused.

Signed-off-by: Martin Schwenke <martin@meltin.net>